and late today another of those killed was identified. 29-year-old krystle campbell of medfordford. we've also learned more about the two bombs planted near the finish line. they were filled with nails and ball bearings designed to maim as many people as possible at an event that drew runners and spectators from all over the world. >> this was a heinous and cowardly act, and given what we now know about what took place, the f.b.i. is investigating it as an act of terrorism. >> pelley: terrorism on patriots' day. a massachusetts holiday that celebrates the opening shots of the american revolution known to many in boston each year as marathon day. and we have just received word that the third person killed was a graduate student at boston university, the president of the university posted an announcement a short time ago, but he did not identify the student. there were reports yesterday that one or more unexploded bombs were disarmed. but it turns out there were only two bombs. both of them exploded. homeland security correspondent bob orr is in washington following the investigation.